During electroplating, a voltage drop variation typically exists along an irregular surface of a substrate which can result in an uneven metal deposit on the substrate. Some parts of the substrate would have been overplated while other parts would have been underplated. This would particularly be a problem when there is a hole on the surface of the substrate. For example, FIG. 1 shows a scanning electron micrograph (“SEM”) of a hole 20 on a substrate having copper 22 electroplated on it by a conventional electroplating process (i.e. electroplating a substrate by submerging it into an electroplating bath without any additives). As shown in FIG. 1, the edge of the hole 20 is overplated with copper 22, while the inner walls of the hole 20 are underplated. As a result, the inner volume of the hole 20 is not properly filled and the resulting conductivity of the substrate plated with copper 22 would be greatly affected.
Levelers have been added into the electroplating bath in order to achieve a uniform metal deposit on a substrate surface. Although the conventional levelers improve the quality of metal deposit done by electroplating on the substrate surface, the quality is still not as desired.